The present invention relates to a continuous casting facility, particularly to a mold level control apparatus for controlling to maintain a molten steel level in a mold constant.
In an operation of a continuous casting facility, it is requested to realize a stable operation and to uniformly maintain cast slab at high quality. For this purpose, in the continuous casting facility, a function of maintaining a molten steel level in a mold constant plays an important role during the whole operation. The function is referred to as a function of "mold level control".
Molten steel accumulated in a tundish is guided from an opening portion disposed at a bottom portion of the tundish, via a pipe referred to as an immersion nozzle, to a rectangle mold. Molten steel injected into the mold is deprived of heat and cooled and an interface between the molten steel and the mold is solidified, a state of the molten steel is changed into slab and the slab is discharged to a downstream side. There is installed a part referred to as a stopper or a sliding gate (hereinafter, referred to as stopper) fabricated by refractory at a bottom portion of the tundish or a portion where the tundish and the immersion nozzle are bonded to each other. The stopper is installed to provide resistance to a flow of the molten steel in the immersion nozzle. The stopper comprises a structure in which the stopper can be moved by drive force such as hydraulic pressure. By adjusting the position of the stopper, a degree of the resistance provided to the molten steel flow in the immersion nozzle is changed. As a result, a flow rate of the molten steel in the immersion nozzle can be adjusted.
Ideally, when a volume per unit time of the slab cast by the continuous casting facility is balanced with the flow rate of the molten steel injected into the mold, the molten steel level in the mold is maintained constant. However, in operating the continuous casting facility, there is a case in which the stopper is melted since the stopper is exposed to the molten steel at a high temperature and a shape thereof is changed. Further, there is a case in which a deposit having a component of the molten steel is adhered to or exfoliated from a portion for passing the molten steel and the molten steel flow rate is changed. Further, there is a case in which an amount of the molten steel accumulated in the tundish is changed and delivery pressure of the molten steel at the opening portion of the immersion nozzle is changed. Further, there is a case in which a deposit of the molten steel is adhered to or exfoliated from the inside of the immersion nozzle and a molten steel guide path of the immersion nozzle is changed.
By these unsteady-state and unpredictable causes, even when the position of the stopper is maintained constant, the molten steel flow rate injected into the mold is changed. Further, the produced slab is discharged to the downstream side in a state in which the slab has not been completely solidified and accordingly, there causes a change in the shape of the slab by bringing the slab into contact with a supporting structure such as a plurality of rolls for transferring the slab. The change constitutes a factor of varying the mold level from the downstream side by flowing back the molten steel in an unsolidified portion in the slab.
In order to maintain the level constant in injecting the molten steel into the mold under such a situation, there is generally adopted a feedback control system as a mold level control system. According to the feedback control system, the mold is installed with a sensor for detecting the molten steel level in the mold. Further, the position of the stopper is adjusted so that an output value of the sensor is made to coincide with a target level provided as an instruction value.
Currently, there has been developed a new process of directly connecting a continuous casting facility with a hot rolling facility and integrally producing hot coils from refined molten steel. This is because there are requests for downsizing a facility scale and accordingly, a reduction in a facility cost as well as high efficiency formation of operation and a reduction in energy required for production. According to a continuous casting facility for playing an important role among these, there is the following request in addition to the above-described requests. That is, with a purpose of alleviating load on a hot rolling process, there are needed properties that a thickness of produced slab is as thin as 80 through 120 (mm) and the slab stays at a comparatively high temperature. In this way, since the slab thickness is comparatively thin, in order to provide priority in production capacity in comparison with that in a conventional facility, there is requested a casting speed as high as 5 through 8 (m/minute) for a continuous casting facility.
Since the slab thickness is thinned and the sectional area of the slab in the mold is smaller than that in the conventional facility, the adjusted molten steel flow rate is reflected sensitively to the mold level. Further, since temperature of the produced slab is high, the slab is extracted to the downstream side in a state in which the slab is softer than that in the conventional facility. As a result, a variation in the shape of the slab on the downstream side is more liable to be caused owing to the contact of the slab with a supporting structure such as rolls and the degree is magnified. As a result, the degree of fluctuation of the mold level derived from the downstream side is increased. Such a variation in the shape of the flexible slab is referred to as unsteady-state bulging. An explanation will be given as follows of serious influence effected by the unsteady-state bulging.
The inventors has constituted a mold level control apparatus by applying PI(Proportional Integral) control which is frequently utilized in a chemical plant as a feedback control system. In that case, when the operation is carried out at a specific casting speed, the inventors have faced a problem that the molten steel level in the mold which has been controlled excellently, abruptly starts oscillating at a frequency of about 0.3 (Hz), the oscillation finally increases and stable operation of the facility cannot be maintained.
When a surface of the cast slab is observed, a nonuniform distribution of the surface condition of the slab which coincides with an interval of rolls can be confirmed in a direction of moving the slab. Further, the oscillation frequency of the mold level substantially coincides with a value of a casting speed divided by the interval between the rolls. The inventors have acquired the knowledge that the oscillatory state of the mold level is caused as follows, from investigation and observation on a number of operational states.
(A) When a flexible slab at a high temperature is supported by the rolls, the slab constitutes a bulged shape in correspondence with the interval between the rolls.
(B) When such slab moves to the downstream side at a constant speed, a spacial nonuniformity of a condition of the slab such as an oscillation mark operates as, for example, frictional force and the slab starts pulsating. When intervals among a plurality of rolls stay equal, degrees of pulsation caused at the respective roll intervals are amplified mutually thereby.
(C) By the above-described pulsation, unsolidified molten steel in the slab flows back to the side of the mold to thereby constitute a periodic disturbance flow which is applied on the mold level. The periodic disturbance flow produces a level fluctuation in the mold at a frequency thereof. Thereby, there is produced a nonuniformity at the above-described period in cooling operation in the mold and the like. As a result, the cast slab is provided with a spacial nonuniformity at the above-described period in surface condition, a composition and the like.
(D) When a portion of the slab having the above-described condition (C) reaches a group of supporting rolls achieving the above-described operation of (B), the pulsation at the above-described period is further magnified. This is operated further to increase the degree of the periodic disturbance flow applied to the mold level. As a result, there is brought about a situation in which the oscillation of the mold level is increased and a stable operation of the continuous casting facility is hindered.
In this way, since the fluctuation in the mold level caused by the unsteady-state bulging of the flexible slab is self-increasing, it is extremely difficult to restrain the fluctuation.
As has been described previously, it is an object of the mold level control apparatus to restrain disturbance applied on an object of the mold level control to thereby maintain the mold level at a predetermined value. Generally, according to a control loop executed in Pi control, when steady-state disturbance in a step-like shape is applied, there is formed an operating amount for canceling the steady-state disturbance (instructed stopper position in the case of the mold level control) by an integration term present in a controller of the control loop. The phase of the integration term is retarded by 90 degrees and therefore, a correcting operation by the integration term is considerably retarded since the disturbance has been received. Further, in the control loop, there are factors for making the mold level control difficult such as delay time of the stopper position control, molten steel drop time and detection delay time of the sensor for detecting the mold level. According to a control object in which the correcting operation is delayed, when the gain of the integration term is set to be large, there is brought about a dangerous state in which the control loop is diverged and accordingly, the disturbance restraining operation in the PI control is limited.
In order to resolve the above-described problem caused by the PI control, conventionally, there have been proposed various mold level control methods (apparatus).
For example, according to Japanese Patent Laid-Open No. 31560/1993 (hereinafter, referred to as Related Art 1), there is disclosed "a level control method in a continuous casting" capable of maintaining the level always stable, swiftly and pertinently in correspondence with all of disturbances. According to the Related Art 1, a feedback control loop operates so that an actual value of the level is made to coincide with a target value of the level. A disturbance canceling control loop predicts a remaining difference amount of disturbance which cannot be feedback-controlled by a feedback control loop by using an instruction value outputted to an actuator, an actual value of the level and a level control model, adds a correcting signal for canceling the remaining difference amount to the instruction value and outputs an added value to the actuator.
According to the Related Art 1, the disturbance canceling loop is set at the inside of a control apparatus, the disturbance applied on an object of the mold level control is predicted and an operating amount is calculated by adding a correcting amount for canceling the predicted remaining difference amount of disturbance. Thereby, the predicted remaining difference amount of disturbance is equivalent to a value produced by differentiating a variation amount by the disturbance in a detected value of the mold level and the apparatus is operated to promptly restrain the fluctuation in the mold level caused by the disturbance.
Further, according to Japanese Patent Laid-Open No. 177321/1993 (hereinafter, referred to as Related Art 2), there is disclosed "a mold level control apparatus" for controlling the level in the mold in a continuous casting process with high accuracy. A control system of a mold level according to the Related Art 2 includes a sliding nozzle or a stopper (hereinafter, referred to as sliding nozzle) for operating an amount of injecting molten steel, a level meter for measuring the molten steel level in the mold and a mold level control apparatus for calculating an opening degree of the sliding nozzle. The mold level control apparatus is provided with a data processing unit for inputting a measured value of the mold level and a set value of the mold level as data input and executing a dynamic compensation calculation at a higher order and a control instruction outputting unit for switching control output after elapse of a predetermined time period.
According to Japanese Patent Laid-Open No. 189009/1993 (hereinafter, referred to as Related Art 3), there is disclosed "a control apparatus" remarkably improving response performance in a control system in which delay time and periodic variation (disturbance) are included. The control apparatus disclosed in the Related Art 3 is a higher order dynamic compensation type control apparatus featured in including a data processing unit for inputting a measured value of the level and a set value of the level as data input and executing a dynamic compensation calculation at a higher order and a control instruction outputting unit for switching the control output after elapse of a predetermined time period.
In the Related Arts 2 and 3, the mixed sensitivity problem in the H.infin. (infinitive) control theory is applied to the mold level control and a mold level controller is constituted by a special linear filter having a higher order number. Thereby, there is provided a controller having a disturbance restraining function more excellent than that in a simple Pi controller. Further, the robust stability of the mold level control loop is ensured by setting an upper limit of process perturbation and ascribing the control to the mixed sensitivity problem.
According to Japanese Patent Publication No. 2598201 (hereinafter, referred to as Related Art 4), there is disclosed "a control apparatus of a level in a mold of a continuous castor" capable of self-controlling, at an early stage, hunting of the level of the molten steel in the mold influenced by a change in a control parameter or the application of the disturbance or the like. The Related Art 4 is a control apparatus in which in producing a cast block by extracting it from a tundish while maintaining constant the level of the molten steel in the mold, an amount of injecting the molten steel into the mold is controlled so that a detected mold level is made close to a target value of the level. According to the control apparatus, there is assumed a controlled state region constituted by a control state comprising a deviation between the target value and the detected value of the level and a first order differential value of the deviation. The control apparatus is provided with a controller, a control gain setting section and an injected molten steel amount controlling section. According to the controller, there can be set a characteristic in which a weighted sum of the deviation and the first order differential value of the deviation is nullified in order to divide a predetermined control state region in the control state region. According to the control gain setting section, the controller is provided with control gains for making control states in the respective control state regions close to the characteristic at the respective control state regions. According to the injected molten steel amount controlling section, the amount of injecting the molten steel is controlled by using the controller based on the control gain of the control state region to which a detected control state pertains.
According to the Related Art 4, the controllers having different control gains which are set so that the weighted sum of the deviation in controlling the mold level and the temporal differential value of the deviation is nullified, are controlled to switch according to the respective control state regions to thereby constitute an application mode of a variable structure control system. The variable structure control system is provided with a preferable property having a high robust performance by constraining a control state onto a stable switch face with a high gain.
According to Japanese Patent Laid-Open No. 79423/1994 (hereinafter, referred to as Related Art 5), there is disclosed "a level control method in a continuous casting" for restraining a fluctuation in the level by realizing stable and excellent level control in respect of unsteady-state disturbance such as bulging or nozzle clogging or exfoliation, parameter error or parameter variation or observed noise which requires a swift response performance.
According to the Related Art 5, there are respectively set a first weight function for reducing a magnitude of a transfer function covering from a disturbance causing level variation to a level control output in a desired frequency region, and a second weight function for reducing a magnitude of a transfer function covering from the disturbance to a point prior to applying the disturbance in a desired frequency region. Further, there are described a state equation and an output equation with the weight functions and control output, a control operating amount and an estimated value of disturbance as state variables. Further, by applying the H.infin. control theory thereto, a feedback calculation signal is calculated and a sum of the feedback calculation signal and the correcting signal for canceling a disturbance amount is applied to an actuator for controlling an amount of flowing the molten metal into the mold. The Related Art 5 is provided with a disturbance amount estimating mechanism similar to the above-described Related Art 1. Accordingly, the control method is operated to cancel the variation amount of the mold level by disturbance. Further, the robust control function of the control loop is promoted by combining with an H.infin. controller.
However, there pose the following problems in the above-described Related Arts 1 through 5.
First, the disturbance estimating mechanisms of the Related Arts 1 and 5 are constituted as observers at a lower order. Therefore, when the process receives a perturbation at a higher order, there is a concern of causing a phenomenon referred to as spill over in which the disturbance estimating mechanism itself oscillates at a high frequency. In order to prevent the spill over, a disturbance estimating gain is obliged to set to be small. Accordingly, a disturbance estimating result is retarded more than an actual disturbance change and the effect of the disturbance estimating mechanisms is limited.
Similarly, also in the case of the Related Art 4, the switch face of the variable structure control system is constituted by a simple linear combination of the control deviation and the temporal differential value of the deviation. Therefore, there is a concern of causing spill over. The spill over of the variable structure control system emerges as a phenomenon in which the control state of the object of the mold level control cannot be constrained onto the switch face and is diverged. Further, in the case of the Related Art 4, the control gains of the controller are switched at short time intervals by reciprocating the control state of the control object at the switch face and there causes a phenomenon of chattering in the temporal transition of the operating amount.
Further, in the cases of the Related Arts 2 and 3, the upper limit of the process perturbation is set and accordingly, the robust stability of the mold level control loop can be ensured and the above-described spill over is not caused. However, the H.infin.controller of the mixed sensitivity problem in which the detected value of the mold level constitutes the input and the instructed position of the stopper constitutes the output, is not so much different from a PID controller which is tuned optimally in view of the frequency characteristic. Therefore, the inventors have an opinion that the control function is not so much different from that of the PID controller which is tuned optimally.
As described above, the fluctuation of the mold level caused by the unsteady-state bulging of the flexible slab is self-increasing and accordingly, it is difficult to restrain the fluctuation by a control apparatus having a more or less disturbance restraining function.